The invention relates to a braking device in a reception station of a machine for processing articles in sheet form.
A machine of this kind usually comprises an insertion station containing a stack of sheets which are successively taken from the top of the stack and conveyed to a feed board. On the feed board, each sheet is positioned against front and side stops before its front edge is gripped by a set of conveying grippers distributed along a transverse bar, the ends of the grippers being secured to lateral drive chains. The conveying grippers entrain the sheets through the various work stations of the machine. The work stations may more particularly be a cutting station followed by a blank ejection station ending at a reception station in which each sheet is released by the conveying grippers and aligned on top of a stack formed on a discharge pallet.
To ensure that the sheet falls uniformly and with the correct alignment, the sheet must be substantially flat when it stops, at the moment when the conveying grippers open. To this end the sheet arriving at the station is first held by a rear bearing surface and, if required, by two lateral bearing surfaces, which are subsequently retracted and drop the sheets.
The sheets are fragile and, after the operation of cutting and ejecting the blanks, form only fragile grids of blanks. Also the grids arrive at the reception station at high speed and if slowed down simply by deceleration of the front gripper bar, the rear part of the sheet may cockle and tend to overtake the front part. The waste grid must therefore be slowed down by a complementary device acting against its surface.
CH patent 689 977 has already proposed a device of this kind, comprising a flexible braking means in the form of a long brush extending across the trajectory of the sheets and mounted so as to be pivotable around a transverse axis, so that its trajectory around the axis intersects the trajectory of the sheets, the direction of rotation of the end of the flexible braking means intersecting the trajectory of the sheets being opposite to the direction of motion thereof. The device comprises drive means for pivoting the braking means in dependence on the longitudinal dimension of the sheets and the frequency with which they pass.
In this device, the motion of the braking means is controlled by a cam connected by a kinematic chain to the mechanism driving the machine. The cam acts on the braking means via a vertically movable horizontal slide, the downstream part of which has an upwardly sloping surface. Since the braking means is mounted on a longitudinally movable frame, its motion remains constant in the case of sheets having a long longitudinal dimension, then decreases progressively in proportion as the frame is moved forward relative to the sloping part of the horizontal slide.
In a device of this kind, the motion of the braking brush is optimum for a given size of sheet, at the expense of the other sizes. The possibility of descent of the braking brush depends on when the sheet-conveying gripper bar passes. The kinematic connection between the brush-lowering means and the mechanism driving the machine limits the acceleration communicated to the braking brush.
The object of the invention is to eliminate the said disadvantages, at least partly.
To this end the invention relates to a programmable braking device in a reception station of a machine for processing articles in sheet form, as described herein.
By means of the device, the motion of the braking means is always optimum in dependence on the size or format and can be optimised in real time in dependence on the speed of motion of the sheets.
Improvement of the braking of the grids for waste is a key factor in avoiding difficulties through jamming. In order to remove the waste grid at high speed without jamming, the grid must be discharged quickly, the next grid must move above the grid being discharged, the braking brush must grip the grid in order to slow it and the grid must be released from the conveying grippers as soon as possible so as to relax the pressure on the braking brush so that the rear of the grid falls on to the conveyor belt.
By means of the device according to the invention, these conditions can be met for each different size of sheet and at all speeds of the machine, since the control means can drive the electric motor at variable speed with a speed curve adapted to each particular case.